Fix default chain def (erincatto#878)

Fixes some sample crashes and should make using the new surface material
easier.

Fix body destruction handling of chain data.

Author: erincatto

src/body.c

@@ -396,8 +396,7 @@ void b2DestroyBody( b2BodyId bodyId )
 	{
 		b2ChainShape* chain = b2ChainShapeArray_Get( &world->chainShapes, chainId );
 
-		b2Free( chain->shapeIndices, chain->count * sizeof( int ) );
-		chain->shapeIndices = NULL;
+		b2FreeChainData( chain );
 
 		// Return chain to free list.
 		b2FreeId( &world->chainIdPool, chainId );

src/types.c

@@ -74,7 +74,13 @@ b2SurfaceMaterial b2DefaultSurfaceMaterial( void )
 
 b2ChainDef b2DefaultChainDef( void )
 {
+	static b2SurfaceMaterial defaultMaterial = {
+		.friction = 0.6f,
+	};
+
 	b2ChainDef def = { 0 };
+	def.materials = &defaultMaterial;
+	def.materialCount = 1;
 	def.filter = b2DefaultFilter();
 	def.internalValue = B2_SECRET_COOKIE;
 	return def;